The disclosure concerns forming screens.
Forming screens are perforated devices that are used to make apertured formed films. Apertured formed films are plastic films that are processed to create apertures or holes in the film. Apertured films fall into two general categories; three-dimensional films and flat films. While all films have three-dimensions in that they have a nominal thickness, a “three-dimensional film” in the context of the present disclosure is one having surface structures that provide a greater thickness to the film beyond the nominal thickness. In general, these surface structures have a memory in that they will resume their shape after being deformed by pressure or tension, so long as the deformation limit of the film has not been compromised. By contrast, flat apertured films are simply films with holes and lack the surface structures with three-dimensional memory that characterize “three-dimensional films”.
There are several processes in the art for making apertured films. Flat films can be made by any process in which holes are added to the film. In general, these are mechanical processes using pins or other embossments to create holes in a precursor film. For three-dimensional films, the two most common processes are vacuum forming and hydroforming. The vacuum forming process, exemplified by references such as U.S. Pat. Nos. 3,939,135 and 4,324,246, uses a vacuum to create a negative pressure on one side of a film and a corresponding positive pressure on the opposite side of the film. The pressure differential causes film to be drawn into a hole in the forming screen to create the apertures. In a hydroforming process, as exemplified by U.S. Pat. No. 4,609,518, high pressure water jets are used to generate streams of liquid that impinge upon the film to create the apertures.
In either the hydroforming or vacuum forming process, the film is supported on a perforated structure known in the art, and referenced herein, as a forming screen. The forming screen may have a textured surface that will translate to the film during the aperturing process. Forming screens are typically made of either metal or plastic. Metal screens have the advantage of being stronger and more robust than plastic screens and are also better able to dissipate heat compared to plastic screens, which means that using metal screens generally permits faster production line speeds. However, metal screens are more expensive than plastic screens. In addition, it is difficult—if not impossible—to create intricate and fine scale designs or structures in the surface of metal screens.
However, while fine scale structures can be formed in plastic screens, maintaining that fine scale texture is a problem. In particular, plastic screens are sensitive to the temperatures of the molten polymer used to make the films in a direct cast process. Because the fine scale textures and structures lack significant mass, they are less able to withstand the heat and the fine scale features can be lost rather easily. In addition, many of the plastic resins used to make the screens are water soluble and thus the screen will degrade over time when used in a hydroforming process because the water will gradually dissolve away the screen.
There is a need in the art for plastic screens that are low in cost, durable, able to accept and maintain fine scale structures and textures even when subjected to high temperatures of molten polymers in a direct cast film process, resistant to degradation in a hydroforming process; and thus suited for use in both direct cast vacuum forming and hydroforming processes.